Method of reducing thermal distortion in grinding machines

ABSTRACT

The invention provides a method of reducing thermal distortion in grinding machines. Such machines each comprise a machine base ( 60 ) and a grinding wheel ( 50 ) for grinding components in the machine ( 10 ). The method includes the steps of: (a) sensing a first temperature at an upper surface of the base ( 60 ) substantially below a position ( 110 ) in the machine ( 10 ) whereat component grinding using the wheel ( 50 ) occurs; (b) sensing a second temperature of an underside surface of the base ( 60 ) substantially below the position ( 110 ) whereat component grinding occurs; (c) determining a relationship between component size drift and changes in a difference between the first and second temperatures; and thereafter (d) correcting a positional offset applied to the wheel ( 50 ) during grinding in accordance with the determined relationship, thereby reducing the component size drift. Preferably, the relationship is substantially a linear function of the form MDS=K f (ΔT 1 −ΔT 2 ) although higher-order polynom be applied if required. The invention also relates to grinding machines ( 10 ) employing the method of reducing thermal distortion.

FIELD OF THE INVENTION

The present invention relates to a method of reducing thermal distortionin grinding machines, such distortion resulting in size drift incomponents manufactured using the machines. The invention relates inparticular, but not exclusively, to a method of reducing thermaldistortion in grinding machines comprising associated machine bases overwhich a coolant flows in operation.

BACKGROUND TO THE INVENTION

It is generally known that components manufactured in grinding machinescomprising associated machine bases can suffer component size drift as aconsequence of small movements and distortions in the bases. In agrinding machine comprising a base where a coolant fluid flows over thebase, it is a logical conclusion that distortions can at least to someextent be affected by the temperature of the fluid.

The inventor has appreciated that size drift in components manufacturedin grinding machines employing associated base cooling can be reduced byapplying a simple correction method to the machines.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provideda method of reducing thermal distortion in a grinding machine comprisinga machine base and a grinding wheel for grinding components in themachine, the method including the steps of:

-   (a) sensing a first temperature at an upper surface of the base    substantially below a position in the machine whereat component    grinding using the wheel occurs;-   (b) sensing a second temperature of an underside surface of the base    substantially below the position whereat component grinding using    the wheel occurs;-   (c) determining a relationship between component size drift and    changes in a difference between the first and second temperatures;    and thereafter-   (d) correcting a positional offset applied to the wheel during    grinding in accordance with the determined relationship, thereby    reducing the component size drift.

The invention provides the advantage that the method is capable ofreducing component size drift.

In practice, a coolant fluid is employed when grinding for cooling andremoving grinding debris. The coolant fluid can be susceptible totransient temperature fluctuations. Thus, preferably, the base includesa panel at its upper surface and the first temperature is measuredwithin the panel away from an upper facing exterior surface of the panelsusceptible to being exposed to the coolant fluid.

In order to ensure that the first temperature is relatively unaffectedby transient temperature fluctuations of the coolant fluid, it ispreferable that the first temperature is measured at a distance in arange of 60% to 90% of the thickness of the panel away from the upperfacing exterior surface.

From detailed studies, the inventor has determined that the firsttemperature is beneficially measured in a coolant fluid gully includedin the machine below the position whereat grinding occurs, the gullybeing included for collecting coolant fluid output from the positionwhereat grinding occurs.

The inventor has found it convenient to measure the first temperatureusing a probe which is substantially thermally isolated from any coolantfluid flowing over the upper surface of the base. When determining ameasurement position, the gully is most appropriate when attemptingthermal distortion correction.

The probe preferably provides an accurate indication of the firsttemperature. Thus, preferably, the first temperature is measured using aprobe which is in thermal communication with the upper surface of thebase by mediation of a heat conductive paste. The inventor has found itespecially convenient to employ a heat conductive paste comprisingsilicone.

Preferably, for convenience of installation and adjustment, the secondtemperature is measured using a probe magnetically attached to theunderside surface of the base.

During studies, the inventor has found it beneficial to achievingeffective thermal distortion correction to arrange for the positionwhereat component grinding occurs and the positions at which the firstand second temperatures are measured to be mutually substantiallyco-linear.

In a general situation, the inventor has found it beneficial to arrangefor the relationship to be of the formMSD=K _(f)(b ₁(ΔT ₁ −ΔT ₂)+b ₂(ΔT ₁ −ΔT ₂)² + . . . +b _(m)(ΔT ₁ −ΔT₂)^(m))wherein MSD is a grinding correction applied, K_(f), b₁ to b_(n) areproportionality constants, ΔT₁ is the first temperature, ΔT₂ is thesecond temperature, and n is a positive integer.

In practice, the inventor has found that the correction required doesnot change abruptly with temperature. Thus, it is preferable that therelationship is a linear, quadratic or cubic function. Such lower-orderfunctions are relatively easy to cope with when calculating theircoefficients from test trials.

The inventor has found it especially appropriate to apply a linearcorrection for thermal distortion. Thus, preferably, the relationship issubstantially a linear function of the formMSD=K _(f)(ΔT ₁ −ΔT ₂)

Moreover, the inventor has evaluated from trials that, for one type ofgrinding machine, the proportionality constant K_(f) is preferably in arange of 25 to 35 μm/° C., namely the constant K_(f) is beneficiallysubstantially 30 μm/° C.

When determining the relationship, it is more complex to characterisethe grinding machine when feedback is applied therearound. Thus, one ormore of the proportionality constants are preferably calculatedempirically from open-loop trials undertaken on the machine wheretemperature correction derived from the base is not applied.

In order to achieve a satisfactory degree of correction, the first andsecond temperatures are measured to a resolution of at least 0.1° C.More preferably, the first and second temperatures are measured to aresolution of at least 0.05° C. If measurement technique allows, it ismost preferable that the first and second temperatures are measured to aresolution of at least 0.01° C.

According to a second aspect of the present invention, there is provideda grinding machine employing the method according to the first aspect ofthe invention, the machine comprising a machine base and a grindingwheel for grinding components in the machine, the machine furthercomprising:

-   (a) first temperature sensing means for sensing a first temperature    at an upper surface of the base substantially below a position in    the machine whereat component grinding using the wheel occurs;-   (b) second temperature sensing means for sensing a second    temperature of an underside surface of the base substantially below    the position whereat component grinding using the wheel occurs; and-   (c) computing means for receiving first and second temperature    measurements from the first and second sensing means respectively    and for calculating therefrom a correction factor for applying to    actuating means for moving the wheel relative to a component to be    ground, thereby reducing component size drift.

Preferably, the machine includes a coolant fluid gully substantiallybelow the position in the machine whereat component grinding occurs, thefirst sensing means being spatially located within the gully. Theinventor has found from studies that the gully is a particularlyappropriate location whereat to measure the first temperature in orderto obtain effective thermal distortion correction.

Conveniently, for ease of installation and adjustment, the secondsensing means comprises a second temperature sensing probe magneticallyattached to the underside surface of the base.

For accurate temperature measurement in an environment whereconsiderable electrical interference is experienced, for example onaccount of the use of electronic motor control equipment, the secondsensing probe preferably includes a platinum resistance thermometer formeasuring the second temperature. Most preferably, the platinumresistance thermometer is a Pt-100 type resistance thermometer.

Preferably, the first sensing means is not directly exposed to coolantfluid flowing on an upper surface of the base as such fluid issusceptible to transient temperature fluctuations. Thus, beneficially,the base comprises an upper panel into which the first sensing means ismounted. More preferably, the first sensing means includes a firstsensing probe which is spatially located within the upper panel and issubstantially thermally isolated from the coolant fluid flowing over theupper surface of the base.

When designing the machine, the inventor has found it beneficial tomount the first sensing means in a blind hole machined into the upperpanel. Preferably, the blind hole is prefilled with heat conductivepaste prior to installing the first sensing means into the hole. Use ofsuch paste assists to ensure that the first sensing means it not greatlyinfluenced by the ingress of coolant fluid and is also in effectivethermal communication with the upper surface of the base. The inventorhas found it especially preferable to employ a conductive pasteincluding silicone, silicone being hydrophobic and thereby repelling thecoolant fluid.

Preferably, the blind hole is bored to a depth corresponding to in arange of 60% to 90% of the thickness of the panel, and the first sensingprobe is mounted substantially at the bottom of the hole. This range iseffective at ensuring that the first probe is sufficiently far from theupper surface of the base where the coolant fluid flows but not so deepthat there is a risk that the hole breaks through into a central regionof the base during base manufacture.

The first probe is preferably mechanically shielded by a carrier forinstallation into the machine. The carrier is preferably a poor heatconductor, otherwise transient temperature fluctuations of the coolantfluid could influence the first probe. Thus, preferably, the first probeis mounted on the carrier for retention in the hole, the carrier beingfabricated from a substantially thermally insulating polymer. Morepreferably, the polymer includes one or more of nylon, polycarbonate,polyethylene, polypropylene and fibre-reinforced phenolic resin. Such apolymer is susceptible to being easily machined or moulded.

For ease of installation and maintenance, the carrier is convenientlyretained by means of a screw thread into the hole.

In order to implement the method of reducing thermal distortionaccording to the first aspect in the machine, a reliable and accuratetemperature measurement is required. Thus, preferably, the first sensingprobe is a platinum resistance thermometer. More preferably, the firstsensing probe is a Pt-100 type platinum resistance thermometer.

The method of the invention according to the first aspect may be appliedto existing grinding machines by way of retro-fitting. Suchretro-fitting involves attaching two temperature sensors and alsoinstalling a software addition to the controlling means to enablecorrection of the grinding wheel offset during use. Preferably, thesoftware addition is operable to provide soft keys for enabling ordisabling component size drift correction according to the first aspectof the invention.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Embodiments of the invention will now be described, by way of exampleonly, in which:

FIG. 1 is an illustration of a grinding machine equipped withtemperature sensing probes according to the invention;

FIG. 2 is a schematic diagram of a temperature sensing probe adapted formeasuring the temperature at an upper surface of a base of the machineillustrated in FIG. 1; and

FIG. 3 is an illustration of a part of the machine shown in FIG. 1.

Referring now to FIG. 1, a grinding machine is indicated generally by10. The machine 10 comprises a headstock 20 and a tailstock 30 betweenwhich an elongate component 40 for grinding is mounted, for example astainless steel tubular component. The machine 10 further comprises agrinding wheel 50 with its associated support structure including one ormore wheel drive motors. Moreover, the machine 10 includes a controlunit (not shown) for controlling its operation, the unit comprising acomputer executing machine control software. The wheel 50 with itsassociated structure, and the tailstock 30 and headstock 20 are allmounted on a base 60 of the machine 10.

Operation of the machine 10 will now be described in overview.

In operation, the control unit actuates the grinding wheel 50 relativeto the component 40. The grinding wheel 50 is rotated and brought intocontact with the component 40 for grinding material therefrom andthereby machining the component 40. When necessary, the component 40 isrotated about its elongate axis A-B so as to bring different parts ofthe component 40 into contact with the wheel 50. A coolant fluid issprayed onto the component 40 and the wheel 50 during grinding, thefluid being collected into a gully 70 in the base 60 for subsequentfiltration and recirculation.

The machine 10 will now be described in further detail.

The machine 10 includes a first temperature probe 100 for measuring afirst temperature of an upper surface of the base 60, the first probe100 being attached to the base 60 below a position indicated by 110whereat grinding of the component 40 occurs. More preferably, the firstprobe 100 is mounted in the gully 70. Moreover, the machine 10 furtherincludes a second temperature probe 120 for measuring a secondtemperature of an underside surface of the base 60, the second probe 120being attached substantially vertically below the first probe 100 andthe position 110 where grinding occurs.

Outputs from the first and second probes 100, 120 are connected to theaforesaid control unit so that its software receives data indicative ofthe temperatures of the upper and underside surfaces of the machine base60.

The second probe 120 is conveniently a magnetically-retained devicewhich is simply applied to the underside surface of the base 60. Thesecond probe 120 can alternatively be bolted or clamped into position onthe underside surface of the base 60. However, the inventor has foundthat the second probe 120 is susceptible to being influenced bytransient air drafts flowing around the underside of the base 60. Inorder to desensitize the second probe 120 from such draughts, theinventor has found it beneficial to include thermal insulation aroundthe probe 120 except where the probe 120 contacts onto the undersidesurface of the base 60. Such thermal insulation can include a clothcover, an expanded-plastics foam cover or simply a plastics materialshell. Thus, the second probe 120 is preferably in intimate thermalcontact with the underside of the base 60 but shielded from airflow onthe underside of the base 60.

In devising the invention, the inventor has had to evolve the design ofthe first probe 100 to make it suitable for mounting onto the uppersurface of the base 60.

Referring now to FIG. 2, there is shown the first probe 100 installedinto a blind hole machined into an upper exterior surface of the base60. The probe 100 comprises a body section 200 fabricated from ahexagonal-top M12 Nylon bolt through which a longitudinal axial hole 210has been moulded or bored. The probe 100 further comprises a temperaturesensing platinum resistance tip 220 connected to a twisted pair of wires230. The tip 220 is located at an end region of the bolt 200 remote froma hexagonal head region 240 of the bolt 200. Moreover, the wires 230 arerouted from the tip 220 through the axial hole 210 to the head region240 wherefrom the wires 230 are further conveyed to the control unit.

Although the bolt 200 is fabricated from nylon, it can alternatively befabricated from another substantially thermally insulating polymer, forexample polypropylene, polyethylene, fabric-reinforced phenolic resin orpolycarbonate. Mixtures of these polymers can also be employed.

The probe 100 is mounted within a blind M12 tapped hole machined intothe upper surface of the base 60 as illustrated. When installing theprobe 100 into the blind hole, it has been found by the inventor to beespecially desirable to prefill the hole with white heat conductivesilicone paste, for example as conventionally employed for improvingheat conductivity from power semiconductor devices such as TO3can-mounted power transistors to associated finned heatsinks. Inclusionof the paste is also effective at reducing the ingress of coolant fluidto the resistance tip 220; the silicone paste is generally hydrophobic.The base 60 is a relatively substantial grinding machine part fabricatedfrom cast iron having an upper surface panel thickness in the order of 3to 5 cm. The blind hole is preferably bored in a range of 60% to 90%through the thickness of the upper surface panel.

The inventor has found that the probe 100 is highly effective atmeasuring the temperature of the upper surface of the base 60 at thesame time as being relatively insensitive to transient temperaturefluctuations of the coolant fluid sprayed during machine operation overthe component 40 and the wheel 50. Such insensitivity is achieved byvirtue of the bolt 200 being fabricated from a substantially thermallyinsulating material. A thermally insulating material is defined as amaterial having a thermal conductivity of less than 1 W m⁻¹ K⁻¹.

When developing the invention, the inventor initially usedmagnetically-retained probes applied to both the upper exterior surfaceof the base 60 and to the underside surface of the base 60 for measuringa temperature difference therebetween. During such development, theinventor found that the magnetically-retained probe applied on the upperexterior surface was rather susceptible to transient temperaturefluctuations in the coolant fluid. Thus, the inventor has appreciatedthat mounting the first probe 100 slightly into the base 60 circumventssuch transient fluctuations, thereby rendering the method of theinvention more accurate and reliable.

The inventor found that temperature differences between the upper andlower surfaces of the base 60 can be relatively small, for example lessthan 1° C. in some grinding situations. Thus, the probes 100, 120preferably have a differential temperature measuring resolution of atleast 0.1° C. More preferable, the probes 100, 120 have a differentialtemperature measuring resolution of at least 0.05° C. Most preferably,the probes 100, 120 have a differential temperature measuring resolutionof at least 0.01° C.

When measuring small temperature differences of 0.5° C. or less, it isconventional practice to employ thermocouples connected in differentialmode. The inventor has found that such a differential arrangement isunsatisfactory because of electrical interference in the environmentsurrounding the machine 10, for example due to high power electronicmotor control equipment. Moreover, although thermistor temperaturesensors are highly sensitive, they are generally insufficiently stableto render them suitable for measuring the temperature difference betweenthe upper surface and underside surface of the base 60. The inventor hastherefore found that platinum resistance thermometers are mostappropriate despite the need to perform an accurate calibration of theprobes 100, 120. The probes 100, 120 preferably employ Pt-100 typeplatinum resistance thermometers.

The inventor has found that, despite including structural features toenhance the rigidity of the base 60, uncompensated grinding accuracy ofthe machine 10 is sensitive to the temperature difference between thefirst and second probes 100, 120. In one example design of grindingmachine, grinding errors of 6 μm were found to correlate withtemperature differences of 0.1° C. between the probes 100, 120.

Referring now to FIG. 3, there is shown a set-up of the machine shown inFIG. 1. The first probe 100 is mounted substantially vertically abovethe second probe 120, the probes 100, 120 being substantially co-linearalong an axis C-D with the grinding region 110.

The inventor has appreciated that the core temperature of the base topsurface directly below the grinding region 110 correlates closely withsize drift when machining the component 40. Moreover, the inventor hasappreciated that the temperature of the underside of the base 60 furtherimproves the degree of correlation, especially when the temperature ofthe base 60 converges towards the temperature of the coolant fluid.

The inventor has further appreciated that an empirical relationshippertains to a diametrical machining size drift (MSD) in μm when grindingthe component 40 and changes in first and second probe temperaturesdenoted by ΔT₁ and ΔT₂ respectively, the relationship as provided inEquation 1 (Eq. 1):MSD=K _(f)(ΔT ₁ −ΔT ₂)  Eq. 1where

-   K_(f)=a proportionality constant (μm/° C.).

For one type of grinding machine modified to include the probes 100, 120as described in the foregoing, a value for the constant K_(f) in a rangeof 25 to 35 was found to be consistently correct; namely, the constantK_(f) preferably has a value of substantially 30. However, this valuefor the constant K_(f) would be expected to change if the type ofgrinding machine were changed.

In grinding machine trials applying a correction as defined by Equation1, improvements in component batch grinding accuracy in a range of 63%to 81% were achieved. Such accuracy improvement was attainable in bothsimulated intermittent and continuous batch grinding conditions.Moreover, the accuracy improvement was attainable irrespective ofwhether or not bedwash and wheel 50 spindle weir coolant had been leftrunning.

Applying the correction defined in Equation 1 to an operating grindingmachine over a period of 3 days was found by the inventor to reducecomponent size drift from 254 μm to 69 μm.

The inventor has appreciated that a dominant effect responsible forgrinding inaccuracy in a grinding machine is attributable to thetemperature of a coolant fluid affecting a base of the machine, and notsignificantly due to any heat generated by the grinding process itselfor from wheel wind-age. It is conventional design practice to route suchcoolant fluid through a gully formed in the base of the machine situatedbelow a region of the machine in which component grinding occurs.

In the foregoing, it will be appreciated that modifications andadditions can be made to the embodiments elucidated in the foregoingwithout departing from the scope of the invention.

For example, although Equation 1 provides a linear relationship betweenthe temperature difference between the first and second probes 100, 120,it is possible for the relationship to be of a more general polynomialform as expressed in Equation 2 (Eq. 2):MSD=K _(f) G(ΔT ₁ ,ΔT ₂)  Eq. 2whereG(ΔT ₁ ,ΔT ₂)=b ₁(ΔT ₁ −ΔT ₂)+b ₂(ΔT ₁ −ΔT ₂)² +b ₃(ΔT ₁ −ΔT ₂)³ +. . .b _(n)(ΔT _(1−Δ) T ₂)^(n)and where

-   b₁, . . . b_(n)=proportionality constants.

Such a more general polynomial form in Equation 2 includes Equation 1within its scope by virtue of the constant b₁ being unity and theconstants b₂ to b_(n) being zero.

The function G is a polynomial function having ΔT₁ and ΔT₂ as inputparameters. Thus, the function G can, for example be a quadraticfunction, a cubic function or an even higher-order function. Moreover,the function G can also be modified to include a time parameter, forexample time from machine switch-on so that warm-up inaccuracies canalso be compensated.

The inventor has found from trials with grinding machines that theconstant K_(f), and the constants b₁ to b_(n) can be determinedempirically from machining accuracy data collated during the trials. Thetrials are best performed with the compensation according to one or moreof Equations 1 and 2 disabled, namely with a grinding machine undertrial operating open-loop with regard to temperature compensationderived from its differential machine base temperature.

If required, data corresponding to machine base temperatures and groundcomponent metrology results, and optionally also time information, canbe input to curve-fitting software executing on a computer fordetermining constants such as the constant K_(f) and the constants b₁ tob_(n) appropriate to employ in, for example, operating software forcontrolling the machine 10 for improving its grinding accuracy.

1. A method of reducing thermal distortion in a grinding machinecomprising a machine base and a grinding wheel for grinding componentsin the machine, the method including the steps of: (a) sensing a firsttemperature at an upper surface of the base substantially below aposition in the machine whereat component grinding using the wheeloccurs; (b) sensing a second temperature of an underside surface of thebase substantially below the position whereat component grinding usingthe wheel occurs; (c) determining a relationship between component sizedrift and changes in a difference between the first and secondtemperatures; and thereafter (d) correcting a positional offset appliedto the wheel during grinding in accordance with the determinedrelationship; thereby reducing the component size drift.
 2. The methodaccording to claim 1, wherein the base includes a panel at its uppersurface and the first temperature is measured within the panel away froman upper facing exterior surface of the panel susceptible to beingexposed to a coolant fluid.
 3. The method according to claim 2, whereinthe first temperature is measured at a distance in a range of 60% to 90%of the thickness of the panel away from the upper facing exteriorsurface.
 4. The method according to claim 1, wherein the firsttemperature is measured using a probe which is substantially thermallyisolated from any coolant fluid flowing over the upper exterior surfaceof the base.
 5. The method according to claim 1, wherein the firsttemperature is measured using a probe which is in thermal communicationby mediation of a heat conductive paste with the upper surface of thebase.
 6. The method according to claim 1, wherein the second temperatureis measured using a probe magnetically attached to the underside surfaceof the base.
 7. The method according to claim 6, wherein the probe usedfor measuring the second temperature is provided with a thermallyinsulating shield to render it insensitive to transient temperaturefluctuations in air flowing over the underside surface of the base. 8.The method according to claim 1, wherein the relationship is of theform: MSD=K_(f)(b₁(ΔT₁−ΔT₂)+b₂(ΔT₁−ΔT₂)²+ . . . +b_(n)(ΔT₁−ΔT₂)^(n)),wherein: (a) MSD is a grinding correction applied; (b) K_(f), b₁ . . .b_(n) are proportionality constants; (c) ΔT₁ is the first temperature;(d) ΔT₂ is the second temperature; and, (e) n is a positive integer. 9.The method according to claim 8, wherein the relationship issubstantially a linear function of the form: MDS=K_(f)(ΔT₁−ΔT₂).
 10. Themethod according to claim 9, wherein the proportionality constant K_(f)is in a range of 25 to 35 μm/° C.
 11. The method according to claim 8,wherein one or more of the proportionality constants are calculatedempirically from open-look trials undertaken on the machine wheretemperature correction derived from the base is not applied.
 12. Themethod according to claim 1, wherein the first and second temperaturesare measured to a resolution of at least 0.1° C.
 13. A grinding machineincluding a machine base and a grinding wheel for grinding components inthe machine, with the use of a coolant fluid, the machine comprising:(a) first temperature sensing means for sensing a first temperature atan upper surface of the base substantially below a position in themachine whereat component grinding using the wheel occurs; (b) secondtemperature sensing means for sensing a second temperature of anunderside surface of the base substantially below the position whereatcomponent grinding using the wheel occurs; and, (c) computing means forreceiving first and second temperature measurements from the first andsecond sensing means respectively and for calculating therefrom acorrection factor for applying to actuating means for moving the wheelrelative to a component to be pound, thereby reducing component sizedrift.
 14. The machine according to claim 13, wherein the machineincludes a coolant fluid gully substantially below the position in themachine whereat component grinding occurs, the first sensing means beingspatially located within the gully.
 15. The machine according to claim13, wherein the second sensing means comprises a second temperaturesensing probe magnetically attached to the underside surface of thebase, and is provided with a thermally insulating shield to render itinsensitive to transient temperature fluctuations in air flow over theunderside surface.
 16. The machine according to claim 13, wherein thebase comprises an upper panel into which the first sensing means ismounted.
 17. The machine according to claim 16, wherein the firstsensing means includes a first sensing probe which is spatially locatedwithin the upper panel and is substantially thermally isolated from anycoolant fluid flowing over the upper surface of the base.
 18. Themachine according to claim 17, wherein the first sensing means ismounted in a blind hole bored to a depth corresponding to 60% to 90% ofthe thickness of the panel and the first sensing probe is mountedsubstantially at the bottom of the hole.
 19. The machine according toclaim 18, wherein the first probe is mounted on a carrier for retentionin the hole, the carrier being fabricated from a substantially thermallyinsulating polymer.
 20. The machine according to claim 13, wherein thefirst sensing probe is a platinum resistance thermometer.